So one way of looking at the Fediverse is this, from what I'm learning...
Forums are commonly about a specific topic, and *only* about a specific topic; off-topic posts are removed or consigned to an off topic board.
Fediverse instances are commonly (not always) about a specific topic, which influences your local/fed timelines (ie "your view into the fediverse")... but *not* only about it.
Not sure if this is correct.